P register indexing

[PANAMATIK]

(10-03-2016 01:00 PM)teenix Wrote:  Just wondering how the full 16 values can be recovered when there are duplicated values in the indexing.

Hi Tony,

I think your question concerns an implementation detail in your HP assembler/emulator.

Only the woodstock HP-21, 22, 25, 27, 19/29C and spice models 31E, 32E, 33E, 33C, 34C, 37E, 38E, 38C and 67/97 use this instruction scheme. The classic models have different opcodes and can set and test all 16 values of p.

p = 14 0074 and 1774 is never used
if p = 4 is always coded as 0044, 1744 is never used
if p # 4 is always coded as 0054, 1754 is never used
if p = 1 is always coded as 0544, 0844 is never used
if p # 1 is always coded as 0554, 0854 is never used
if p = 14 is never used, no opcode available (only in classic models)
if p # 14 is never used, no opcode available (only in classic models)

From all above models there is only one, that uses the instruction p = 14 coded as 0074 at address 0-5777 (HP-33C) .
(Correction: this address is a data word, not part of the code and thus never will be executed)

As p is used for indexing 1 out of 14 bytes of the c register, its range is normally from 0-13, it could be designed as a settable synchronous 4-bit up/down counter which wraps at 14, but the real implementation in the ACT chip is unknown.

I think Eric Smith did run a ROM emulator when he developed the "nonpareil" emulator, which injected also the 1774, 1744, 1754, 0844, 0854 opcodes and he defined the behaviour of these unused opcodes in his documentation. From HPs point of view they are undefined instructions.

Bernhard